Devices for converting optical power into electrical power are used in many applications, perhaps most known of which is the conversion of sunlight into electricity, for which silicon-based photovoltaic cells, also referred to as photocells or solar cells, are typically used. For this type of application, an array of photovoltaic cells with a relatively large total photosensitive area is typically used.
Another application for optical power converters that has recently emerged is a remote optical powering of electronic modules. In this type of applications, optical power may be provided by a high-power laser and delivered to the module via an optical fiber, where it is converted into an electrical power signal. In such applications, the optical light is typically comes in the form of a well-confined infra-red beam of high intensity, typically in the wavelength range between about 900 nm and 1600 nm corresponding to a low absorption window of typical optical fibers. The optical power converter for such applications has a relatively small, preferably circular photosensitive area of a few square millimeters or less, and is based on compound semiconductors such as InP, GaAs and their alloys, that are more suitable for absorbing infra-red light than silicon. An example of such a GaAs device is disclosed in U.S. Pat. No. 5,342,451 issued to Virshup, which is incorporated herein by reference.
As described in the Virshup patent and illustrated in FIG.1, which reproduces FIG. 2 of Virshup, a typical semiconductor optical power converter includes a pn junction 20 formed between a base layer 16, which may typically be n-type, and an emitter layer 18, which may typically be p-type. The base and emitter layers 16, 18 are sandwiched between a highly conductive buffer layer 14 placed on an isolating substrate 12, and a highly conductive, optically transparent window layer 22, which can be followed by a thicker conducting layer 24. A cap layer (not shown in FIG. 1) is placed on top of the window layer and electrical contacts (not shown in FIG. 1) such as metal grid lines are deposited onto this cap layer. Between the metal grid lines, which are typically as narrow as 3 microns, the cap layer is etched away, allowing the incident light to penetrate through the transparent window layer and be absorbed by the underlying pn junction 20. In order to complete the circuit, the n-portion of the pn junction has to be reached, typically by an etching technique. The depth of this etch is usually a few microns to ensure that the n-type layer 16 is reached. Once metal has been deposited onto this exposed n-type layer, connection to both the n-type and the p-type polarities can be made. As photons are incident and absorbed by the pn junction 20, carriers are generated, and, in the presence of the electrical field created by the pn junction, swept away and collected at the electrical contacts, creating a voltage difference between them. By connecting to an external circuit, electrical power can be extracted.
An open circuit voltage that a single pn junction can deliver is limited by the bandgap of the used semiconductor material, and is typically less than 1 volt. By connecting a number of these pn junctions in series in a lateral configuration, individual voltages are summed to produce an output voltage that can reach 12 volts or more. In order to create such individual pn junction elements, etching between adjacent segments is typically done. This has to be deep enough to isolate the pn junctions from each other, with a typical depth of about 25 microns. To complete the series connections, air bridges are formed between adjacent pn junctions. One side of such an air bridge is connected to the exposed n-type portion of the pn junction; the other to the p-type portion of the neighbouring pn junction.
For applications wherein the optical power is delivered by a confined optical beam with a circular cross-section, such as that emanating from an optical fiber. As illustrated in FIG. 2, which is reproduced from FIG. 3 of the Virshup patent, these multiple pn junctions are typically created by dividing a circular photosensitive area of a semiconductor chip into a plurality of pie-shaped device segments 28 that are separated from each other by etched trenches 26, and connected in series by the interconnecting bridges 40; although 6 segments are shown, in a typical device there may be anywhere from two to sixteen or more such segments.
One drawback of the device shown in FIG. 2 is that it has a hole in the center of the photosensitive structure where the trenches 26 converge; accordingly, light that impinging the device center is lost. Generally, the efficiency with which light is converted into electricity by each portion of the device is proportional to the ratio of the photosensitive area to the trench area; in the device shown in FIG. 2, this ratio has a maximum at the periphery of the photosensitive area, gradually decreasing towards the device center 99, and is substantially zero in the device center 99. Since a typical light beam, such as a beam emanating from an optical fiber, has an approximately Gaussian power distribution with a maximum intensity at the center of the beam, the pie-shaped segmentation illustrated in FIG. 2 results in the loss of a significant portion of incoming light, and therefore negatively affects the overall power conversion efficiency of the device.
A further drawback of the prior art is also related to the non-uniform distribution of the optical power across the light beam that is incident on the power converter device. With so much of the light energy striking a relatively small area about the center 99 of the power converter, many of the photo-generated carriers are generated far from the electrical contacts 40 that are located at the device edges; as a result, an internal resistance of the device layers starts to play a significant role as the total light intensity increases. The increased current is a direct result of the increased light intensity. This current has to make its way through resistive material to the edges, and the more current the more voltage losses caused by the current encountering resistance. With increasing light intensity, the internal resistance of the device results in a voltage drop within the device as the photo-generated current increases, thereby reducing the output device voltage and the electrical power that the device can provide to external circuitry. Consequently, the higher the light intensity, the higher the loss in the output voltage typically observed for the prior art semiconductor power converters.
Another disadvantage of the prior art power converters is that they are geared exclusively for power conversion, while real-life applications may require to combine both optical power conversion and optical data extraction in one module. However, the tasks of optical power conversion and data extraction from modulated light beams result in contradictory design requirements when applied to the prior-art devices such as that illustrated in FIG. 2; for example, a typical requirement in optical power conversion is to maximize the converted power, which demands using devices with relatively large photosensitive area. On the other hand, high-speed optical data communications, for example those with modulation bandwidth in excess of 0.1-1 MHz, will typically require the use of small-area photodetectors that have low parasitic capacitance. The modulation bandwidth of the prior art segmented optical converter of FIG. 2 can be further decreased due to the serial connection of the device segments, which increases the internal device resistance resulting in higher RC values.
An object of the present invention is to overcome at least some of the shortcomings of the prior art by providing an improved optical power converter that has an improved light conversion efficiency for Gaussian beams and lower internal resistance.
Another object of the present invention is to provide an optical power converter that can be used for both the optical power conversion and data detection.